.container {
  width: 100%;
  height: 100%;
  background: pink;
}

.container div {
  width: 400px;
  height: 400px;
}

.content {
  background: blue;
}

.new {
  z-index: -100;
  position: fixed;
  top: 50%;
  left: 50%;
  margin-top: -200px;
  margin-left: -200px;
  background: red;
  border: 10px solid red;
}

.edit {
  z-index: -100;
  position: fixed;
  top: 50%;
  left: 50%;
  margin-top: -200px;
  margin-left: -200px;
  background: yellow;
  border: 10px solid yellow;
}
.mask {
  z-index: 999;
  background: #ccc;
}